Interactive effects of nutrient heterogeneity and competition: implications for root foraging theory?

1. Plants are known to respond to heterogeneous distribution of nutrients in the soil, and they also respond to the presence of neighbouring roots. However, it is unclear whether plants are able to distinguish between these factors and adjust their root responses accordingly. 2. We investigated whether the simultaneous response to nutrient heterogeneity and competition could be predicted from the responses to these factors separately. As a null model, we hypothesized that the responses to nutrients and competition are additive and thus no interactions occur. We performed a short-term competition experiment in the greenhouse with two floodplain species in homogeneous and heterogeneous conditions. The consequences of different root distributions for nutrient uptake were tested using 15N pulse-labelling. 3. Both species responded to nutrient heterogeneity by investing significantly more roots in the nutrient-rich patch, and both species showed a significant reduction in root growth in response to competition, albeit that the reduction was much more pronounced for the grass species. For Rumex palustris, the effects of heterogeneity and competition were additive. However, the response to nutrient heterogeneity of Agrostis stolonifera was reversed by competition: instead of proliferating in the nutrient-rich patch, it significantly increased root investments in the ‘empty’ (nutrient-poor) patches. As the partitioning of total N was less asymmetric than 15N uptake from the nutrient-rich patch, it appears that these altered root investments of A. stolonifera in the ‘empty’ patches have also been functional with respect to compensating N uptake. 4. Our results suggest that root responses to nutrient distribution in a competitive environment depend on the competitive strength of the neighbouring species. The foraging response of the superior species (R. palustris) was hardly affected, but that of the inferior species (A. stolonifera) was greatly inhibited and even reversed by competition: instead of proliferating in the nutrientrich patch, it increased root growth and foraging activity in less favourable patches. Incorporating competitive hierarchy into root foraging studies may help to explain the ambiguous results found in previous studies. Key-words: Agrostis stolonifera, below-ground competition, competitive hierarchy, 15N labelling, root foraging, Rumex palustris, selective root placement, soil heterogeneity